1. Field of the Invention
The present invention relates to a contact structure for electrically connecting together conducting lines formed on two opposite substrates, respectively, via conducting spacers and, more particularly, to a contact structure used in common contacts of an electrooptical device such as a liquid crystal display.
2. Description of the Related Art
In recent years, liquid crystal displays have been extensively used in the display portions of mobile intelligent terminals such as mobile computers and portable telephones including PHS (personal handyphone system). Also, active-matrix liquid crystal displays using TFTs as switching elements are well known.
A liquid crystal display comprises two substrates and a liquid crystal material sealed between them. Electrodes are formed on these two substrates to set up electric fields. A desired image or pattern is displayed by controlling the magnitudes of these electric fields. In the active-matrix liquid crystal display, TFTs (thin-film transistors) are formed on one substrate to control the supply of voltage to each pixel electrode. Therefore, this substrate is referred to as the TFT substrate. A counter electrode placed opposite to the pixel electrodes is formed on the other substrate and so it is referred to as the counter substrate.
In the active matrix display, an electric field is produced between each pixel electrode on the TFT substrate and the counter electrode on the counter substrate, thus providing a display. The potential at each pixel electrode on the TFT substrate is controlled by the TFT and thus is varied. On the other hand, the counter electrode on the counter substrate is clamped at a common potential. For this purpose, the counter electrode is connected with an extractor terminal via a common contact formed on the TFT substrate. This extractor terminal is connected with an external power supply. This connection structure clamps the counter electrode at the common potential.
The structure of the common contact of the prior art active-matrix liquid crystal display is next described briefly by referring to FIGS. 12-14.
FIG. 12 is a top plan view of a TFT substrate 10. This TFT substrate comprises a substrate 11 having a pixel region 12, a scanning line driver circuit 13, and a signal line driver circuit 14. In the pixel region 12, pixel electrodes and TFTs connected with the pixel electrodes are arranged in rows and columns. The scanning line driver circuit 13 controls the timing at which each TFT is turned on and off. The signal line driver circuit 14 supplies image data to the pixel electrodes. Furthermore, there are extractor terminals 15 to supply electric power and control signals from the outside. The substrate 11 makes connection with the counter electrode at common contact portions 16a-16d.
FIG. 13 is a cross-sectional view of the pixel region 12 and a common contact portion 16 representing the common contact portions 16a-16d. A TFT 17 and many other TFTs (not shown) are fabricated in the pixel region 12 on the substrate 11. An interlayer dielectric film 18 is deposited on the TFT 17. A pixel electrode 19 connected with the drain electrode of the TFT 17 is formed on the interlayer dielectric film 18.
A precursor for the source and drain electrodes of the TFT 17 is patterned into internal conducting lines 21 at the common contact portion 16. The interlayer dielectric film 18 is provided with a rectangular opening. A conducting pad 22 is formed in this opening and connected with the internal conducting lines 21. The pixel electrode 19 and the conducting pad 22 are patterned from the same starting film.
FIG. 14 is a top plan view of the known common contact portion 16. A region located inside the conducting pad 22 and indicated by the broken line corresponds to the opening formed in the interlayer dielectric film 18.
As shown in FIG. 13, a counter electrode 24 consisting of a transparent conducting film is formed on the surface of a counter substrate 23. This counter electrode 24 is opposite to the pixel electrodes 19 in the pixel region 12 and to the conducting pad 22 at the common contact portion 16.
Spherical insulating spacers 25 are located in the pixel region 12 to maintain the spacing between the substrates 11 and 23. A spherical conducting spacer 26 is positioned at the common contact portion 16 and electrically connects the counter electrode 24 with the conducting pad 22. The pad 22 is electrically connected with the internal conducting lines 21, which in turn are electrically connected with an extractor terminal 15. This connection structure connects the counter electrode 24 on the counter substrate 23 with the extractor terminal 15 on the substrate 11.
In the prior art liquid crystal display, the interlayer dielectric film 18 is provided with the opening at the common contact portion 16, as shown in FIG. 13. Therefore, the cell gap G.sub.c in the common contact portion is almost equal to the sum of the cell gap Gp in the pixel region+the film thickness t of the interlayer dielectric film 18.
The cell gap Gp (also known as the cell spacing) in the pixel region 12 is determined by the insulating spacers 25. It is common practice to use standardized spacers as the insulating spacers 25 and so if the spacers 25 have a uniform diameter, the cell gap Gp in the pixel region 12 is substantially uniform among liquid-crystal cells. However, it is difficult to avoid nonuniformity of the cell gap G.sub.c in the common contact portion among liquid-crystal cells.
The cell gap G.sub.c in the common contact portion is constant since the cell gap Gp is constant because of the relation described above. Therefore, the cell gap G.sub.c in the common contact portion depends only on the film thickness t of the interlayer dielectric film 18. Consequently, to make the cell gap G.sub.c uniform among liquid-crystal cells, it is necessary that the film thickness t of this interlayer dielectric film 18 be uniform among cells. However, this is impossible to circumvent.
Normally, the common contact portions of the liquid crystal display are 2 to 4 in number. The film thickness t of the interlayer dielectric film 18 may differ from location to location on the same substrate. In this case, the film thickness t may differ among different common contacts even on the same substrate.
Because of the aforementioned nonuniformity of the thickness t of the interlayer dielectric film 18, the cell gap G.sub.c in the common contact portion differs among different cells or different common contacts. Furthermore, the nonuniformity of the cell gap G.sub.c results in the cell gap Gp in the pixel region to be nonuniform.
The cell gap Gp in the pixel region is affected more by the nonuniformity of the cell gap G.sub.c in the common contact portion as the area of the pixel region 12 becomes narrower than the area of the common contact portion. Especially, in the case of a projection display as used in a projector, the problem of above-described nonuniformity of the cell gap Gp in the pixel region becomes conspicuous, because it is a quite accurate small-sized display of about 1 to 2 inches.
A standardized spacer is also used as the conducting spacer 26. The diameter of this conducting spacer 26 is determined by the diameter of the insulating spacers 25 in the pixel region 12 and by the design thickness of the interlayer dielectric film 18. Where the thickness of the interlayer dielectric film 18 is much larger than the designed value, the cell gap G.sub.c in the common contact portion becomes very large. This makes it impossible to connect the counter electrode with the conducting pad well by the conducting spacer 26. In consequence, the counter electrode cannot be clamped at the common potential. As a result, a display cannot be provided.